Presently, glass fibers are formed into a mat material, somewhat like a fabric. This material has a variety of uses. For example, it may be later mixed with a synthetic resin to form a strong, durable fiberglass material or it may be used as a filter.
The general method presently being used to form glass fiber mat comprises projecting glass fibers from a cylindrical drum onto a laterally moving conveyer to form layers of fibers arranged in a generally random, horizontal fashion. These fibers and fiber layers are later bonded together to form a mat.
The industry which uses this mat in its products has begun to require a more homogeneous and uniform pattern of fibers in the mat than the randomly oriented fibers in the mat produced by the conventional methods. This is due to the fact that the products which are formed from the glass fiber mat and the processes by which they are formed have become more precise and thus tolerate fewer inconsistencies and flaws in the materials used.
The characteristics of conventionally manufactured mat are generally random and uncontrollable. One portion of the mat may have very high strength while another portion may have very low strength or even be devoid of mat. T h i s randomness and uncontrollability leads to mat which does not perform predictably in forming processes and which leads to faulty products.
Controlling the directionality of the strength of glass fiber mat is crucial. This is because the different manufacturing methods of today require many different strength characteristics. Where one process may require equal strength in all directions, another may require as much strength as possible in one direction with a considerably lesser requirement of strength in the opposite direction. For example, a ladder rail should have greater strength longitudinally than it has laterally, but some lateral strength is also necessary. Products like this are typically formed by a pultrusion process.
Pultrusion is a process in which glass fiber roving and glass fiber mat are pulled first through a resin bath, then into a forming die and then cured through a number of possible processes. This is similar to extrusion in its final product, but is used with fiberglass instead of metal. The roving provides the principal longitudinal strength reinforcement but provides essentially no lateral strength reinforcement. Consequently, the glass fiber mat is used to supply the necessary lateral strength. Thus, since the purpose of the mat is to provide the lateral reinforcement, pultrusion requires a mat which will provide a high degree of strength in the lateral direction of the finished product. Pultrusion also requires that a sufficient quantity of resin be drawn from the bath into the die with the glass fiber mat.
Match metal die molding, where two interlocking molds press and cure a mat and resin, requires as well that the glass fiber mat retain a certain amount of resin but with the mat having strength characteristics equal in all directions, that is, omni-directional strength.
Other processes in which resin is injected into an assembled mold, through the glass fiber mat, require similar mat characteristics to match metal die molding.
All of the previously mentioned processes require that the mat be uniform in its arrangement of the fibers throughout the mat for predictable performance characteristics. The methods which are presently used to form glass fiber mats do not provide sufficient control over the dimensions or orientation of the layers of glass fibers. This control is required to ensure the uniformity of the glass fiber mat and therefore the predictability of the performance of the finished product formed from the mat.
Another characteristic of the mat, which plays an important role in the highly precise processes, is the ability of the mat to pick up and transport within it a sufficient quantity of resin. This ability is a function of fiber surface area, mat density and the spacing between the fibers. These factors are important in determining the amount of resin carried into the forming dies, or held in the dies by the mat. It is therefore desirable to have control over them.
The highly precise processes and the precision they require create a need for a mat pattern control method which gives control over critical parameters of manufacture and which increase uniformity of the final product.